Inisheer in July 2012
See some of Inisheer's top sights with this six-stage multicache. It is best done by bicycle, which can be rented for a small fee near the pier, but your two legs will work just as well (if a littler slower!)
At each of the five prescribed locations (coordinates for each are found in the "Additional Waypoints" section) you will collect information which will assist you in calculating the final coordinates. Note: the given coordinates are for Stage 1.
STAGE 1: On the sign found at this sacred location, there is a two-digit number in the fourth line of text. Let the first digit be C and the second digit D. Now divide the two-digit number by 2. Let this number be H.
STAGE 2: The largest numbers on the island can be seen from this location. Let the second digit of this two-digit number be E.
STAGE 3: From the coordinates find the name painted in graffiti just to the stern side of the smokestack, underneath the overhang (hint--it begins with "A"). Count the letters in this name and divide by 2. Let this number be A. Now look at the last letter of the name. Figure out the number that represents its chronological order in the alphabet, and let this number be F (for example, A=1, B=2, Z=26, etc).
STAGE 4: This spot is as close as you can get to the lighthouse (suprisingly, it's private property). On the stone post with gold lettering, there is a four digit year. Let the third digit be B.
STAGE 5: Count the number of doors through which this ruin can be entered from the ground level. Add three to this number, and let it be G.
FINAL CACHE: Plug the numbers you have found into the coordinates below to calculate the final cache location. The cache is a prefab bottle cylinder containing a pencil and log, and is a short walk from the pier.
N 53 0A.BCD
W 009 3E.FGH
Good luck, happy hunting, and enjoy Inisheer (I did)!
--IntotheWoods 
Big thanks to SaaX for providing the cache, and for maintaining it for me!